1. Field of the Invention
The present invention relates to an electroluminescence display device, and more particularly, to an organic electroluminescence display device and a method of fabricating the same.
2. Discussion of the Related Art
Cathode ray tube displays have been widely used as display devices for televisions and computer monitors. However, cathode ray tubes have large size, large weight, and high driving voltage. Therefore, flat panel displays—which are thin, light weight, and low in power consumption—have been in demand. Such flat panel displays include liquid crystal display devices, plasma display panel devices, field emission display devices, and electroluminescence display devices.
The electroluminescence display device may be categorized into inorganic electroluminescence display devices and organic electroluminescence display devices depending upon the source material for exciting carriers. Organic electroluminescence display devices have drawn considerable attention due to their high brightness, low driving voltage, and natural color images throughout the entire visible light range. Additionally, organic electroluminescence display devices have superior contrast ratio because of their self-luminescence. Organic electroluminescence display devices can easily display moving images due to its short response time of several microseconds and are not limited by a specific viewing angle. Organic electroluminescence display devices are stable at low temperatures, and their driving circuit can be easily fabricated because they are driven by a low voltage. In addition, the manufacturing process for organic electroluminescence display devices is relatively simple.
In general, an organic electroluminescence display device emits light by injecting electrons from a cathode electrode and holes from an anode electrode into an emissive layer. The electrons combine with the holes, thereby generating an exciton. The exciton then transitions from an excited state to a ground state to emit light. Since its luminous mechanism is similar to a light emitting diode, the organic electroluminescence display device may be called an organic light emitting diode (OLED).
FIG. 1 shows a band diagram of a related art organic electroluminescence display device. As shown in FIG. 1, the related art organic electroluminescence display device includes an anode electrode 1, a cathode electrode 7, a hole transporting layer 3, an emissive layer 4, and an electron transporting layer 5 disposed between the anode electrode 1 and the cathode electrode 7. The related art organic electroluminescence display device further includes a hole injection layer 2 disposed between the anode electrode 1 and the hole transporting layer 3, and an electron injection layer 6 disposed between the cathode electrode 7 and the electron transporting layer 5. The hole and electron injection layers 2 and 6 facilitate efficient injection of holes and electrons, respectively.
The holes are injected into the emissive layer 4 through the hole injection layer 2 and the hole transporting layer 3 from the anode electrode 1. The electrons are injected into the emissive layer 4 through the electron injection layer 7 and the electron transporting layer 5 from the cathode electrode 7. Together, a hole and an electron generate an exciton 8 in the emissive layer 4. Then, light corresponding to energy between the hole and the electron is emitted from the exciton 8.
The anode electrode 1 is formed of a transparent conductive material having a relatively high work function, such as indium-tin-oxide. The light from the electroluminescence display device is observed at the anode electrode 1. On the other hand, the cathode electrode 7 is formed of an opaque conductive material having a relatively low work function, such as aluminum, calcium, and aluminum alloy.
FIG. 2 is a plan view of a related art organic electroluminescence display device. As shown in FIG. 2, a plurality of first electrodes 12 are formed horizontally on a transparent substrate 10, and a plurality of second electrodes 13 are formed vertically on the transparent substrate 10. The plurality of first electrodes 12 and the plurality of second electrodes 13 cross each other to form a matrix. An organic layer 14 is formed between the first electrodes 12 and the second electrodes 13.
FIG. 3 is a cross-sectional view of showing a portion of the related art organic electroluminescence display device of FIG. 2. As shown in FIG. 3, the first electrode 12 is formed on the transparent substrate 10, and a hole injection layer 15 is formed on the first electrode 12. An organic emissive layer 17 is formed on the hole injection layer 15, and an electron injection layer 19 is formed on the organic emissive layer 17. The second electrode 13 is formed on the electron injection layer 19. As stated above, although not shown in FIG. 3, a hole transporting layer and an electron transporting layer may be formed between the hole injection layer 15 and the organic emissive layer 17 as well as between the organic emissive layer 17 and the electron injection layer 19, respectively.
The first electrode 12 is made of a transparent conductive material such as indium-tin-oxide (ITO), and injects holes into the organic emissive layer 17 through the hole injection layer 15. Therefore, the first electrode 12 is an anode electrode. The second electrode 13 injects electrons into the organic emissive layer 17 through the electron injection layer 19. Thus, the second electrode 13 is a cathode electrode.
The organic emissive layer 17 is made of a polymer or a monomer. An organic emissive layer made of the monomer has a high purity. However, it is difficult to form the organic emissive layer of a monomer because a depositing method, such as a chemical vapor deposition or a thermal deposition, is needed. Furthermore, the organic emissive layer made of the monomer requires a high driving voltage, and is not thermally stable.
On the other hand, because an organic emissive layer made of the polymer is formed by an inkjet method or a coating method, such as a spin coating or a roll coating, the organic emissive layer made of the polymer is easily manufactured. The organic emissive layer made of the polymer has a high thermal stability. In addition, a low driving voltage can be used to drive a polymer organic emissive layer.
Generally, in the roll coating method, the polymer is dissolved in a solvent and is coated on a substrate as a mixture by using a roller. Next, the solvent is evaporated, and thus, only the polymer remains. The solvent is made of a material that has a low boiling point and is easily evaporated. Xylenes or O-Dichloro Benzene are widely used as solvents. However, since Xylenes have a boiling point of about 145 degrees centigrade and O-Dichloro Benzene has a boiling point of about 180 degrees centigrade, Xylenes and O-Dichloro Benzene are easily evaporated. Therefore, solubility of the polymer is lowered such that the organic emissive layer coated on the substrate has bad properties.
FIG. 4 is a scanning electron microscope (SEM) photograph showing a surface of an organic emissive layer formed by using O-Dichloro Benzene as a solvent. Here, an evaporation rate of O-Dichloro Benzene is about 0.15 fractional weight per minute. As shown in FIG. 4, the surface of the organic emissive layer has grains of various sizes, which are in a tangle.
In the alternative, Ethyl Benzoate or 2-Butoxy Ethanol may be used as the solvent. FIGS. 5 and 6 are scanning electron microscope (SEM) photographs showing a surface of an organic emissive layer according to the related art.
In FIG. 5, Ethyl Benzoate is used as a solvent. Ethyl Benzoate has a high surface tension of about 34 dyne/cm. As shown in FIG. 5, the organic emissive layer has poor coating characteristics due to the high surface tension.
In FIG. 6, 2-Butoxy Ethanol is used as a solvent in a mixture together with a polymer. Because the solubility of the polymer in 2-Butoxy Ethanol is less than about 1 wt. %, the polymer or organic emissive material is precipitated during the roll coating. Thus, the organic emissive layer has poor characteristics due to the low solubility.